OMG Yes! Please let them make appearances, please, please, please.
As for Nanny's new voice, Jenny Slate seems like a very... Interesting choice. She's a casual, somewhat over the top comedian, in contrast to Nanny's original voice Barbara Billingsly who was literally famous for being motherly (she played Beaver's mom on Leave it to Beaver). But, i'm intrigued to see what direction Jenny and the writers take the character.
Jenny "Smellwether" Slate has kind of a squeaky, youthful voice to her, more like that of a teen or college-aged babysitter as opposed to a June Cleaver type. So I like the direction they're going with.
Also, Nanny seems to have ditched her trademark striped stockings, or at least varied her wardrobe beyond that. Notice the dragon pattern in the first shot, and airplanes in the next, which could relate to the theme of the episode. Reminds me of Miss Frizzle and her dresses.